A fire caused by a heating system broke out in a portable classroom at George Washington Carver Elementary School Annex in Great Mills this week, damaging three buildings and temporarily closing the school, authorities said.

No one was injured in the blaze, which started about 11:15 Monday night when a forced-air heating system attached to the outside of a portable classroom ignited the wood framing of the building, said W. Faron Taylor, deputy state fire marshal. The combustion was the result of pyrolysis, Taylor said, meaning the wood dried and degraded over time until the heat was enough to set it ablaze.

"This is a very rare phenomenon," Taylor said, adding that low humidity over a long time can contribute to the process.

The fire started in the art room of a two-classroom trailer and spread by covered walkway to other buildings, said J. Bradley Clements, St. Mary's County public schools chief administrative officer. One trailer was destroyed, another suffered moderate damage and a third had minor damage to the exterior, the fire marshal's office said. The fire caused an estimated $400,000 in damage.

Classes at Carver Annex, which is temporarily housing students until construction of the new Carver Elementary School is completed, were canceled Tuesday and yesterday. Clements said the school would reopen today. A replacement three-classroom trailer, originally purchased by the school system for use at Benjamin Banneker Elementary School in Loveville, arrived at Carver Annex on Tuesday, Clements said, adding that the structure would be able to house classes by next week.

"They'll only be in tight quarters for a couple days," Clements said.

School system officials met with Carver teachers Tuesday to discuss a response to the fire, and a meeting with parents was scheduled last night at the school. Clements said that starting this week, the school system will inspect all buildings that use the same type of forced-air heating system.

About 50 firefighters from the Bay District, Leonardtown, Ridge, Hollywood and Patuxent River departments responded to the blaze. It took about 40 minutes to control, according to the fire marshal's office.

St. Mary's County Commissioner Thomas A. Mattingly Sr. (D-Leonardtown), a volunteer firefighter in Leonardtown, also responded to the fire.

"They had a fair amount of fire when they first got there. The units that caught on fire were actually in the center of the complex," he said. "It was a difficult spot to get into."

The main building formerly housed community college operations and also was used by students when Lexington Park Elementary School was being renovated, Mattingly said.

The new Carver school is expected to be completed in time for the beginning of classes in August, Clements said.